In modern integrated circuits, semiconductor devices are formed on semiconductor substrates, and are connected through metallization layers. The metallization layers are connected to the semiconductor devices through contacts, also referred to as contact plugs. Also, external pads are connected to the semiconductor devices through the contacts.
Typically, the formation process of contacts includes forming an inter-layer dielectric (ILD) over semiconductor devices, forming contact openings in the ILD, and filling a metallic material in the contact openings. However, with the increasing down-scaling of integrated circuits, the above-discussed processes experience shortcomings. While the horizontal dimensions, such as the widths of contacts, are continuously shrinking, the thickness of the ILD is not reduced accordingly to the same scale as the widths. Accordingly, the aspect ratios of contacts continuously increase, causing the contact formation process to be more and more difficult.
The difficulty is further increased for forming contacts to Fin field-effect transistors (FinFET). FinFETs are widely used since they use less chip area. FIG. 1 illustrates a FinFET structure. Gate 4 crosses three fins 2, and the portions of fins 2 not covered by gate 4 are used to form source and drain regions. The source regions of the three FinFETs are interconnected, and the drain regions of the FinFETs are interconnected. Therefore, the three FinFETs act as a single FinFET. The introduction of FinFETs has the advantageous feature of increasing drive current without the cost of occupying more chip area. However, it is difficult to form contacts to fins 2. Since the fins 2 of the FinFETs have small dimensions, the landing areas for the corresponding contacts are small. To ensure that contacts land on the fins, the landing areas need to be increased. One of the previous solutions is shown in FIG. 1, wherein landing pads 6 are formed and connected to fins 2. Landing pads 6 may be formed of the same material, and by the same process, as fins 2. The contacts to the source and drain regions can thus be easily landed on landing pads 6. However, landing pads 6 themselves occupy chip area, and hence such a solution defeats the purpose of forming FinFETs, which is to save chip area.
Accordingly, what is needed in the art is a contact structure and formation methods for solving the above-discussed problems.